Capacity coupled constant amplification circuits



Dec. 22, 1936.

CAPACITY W. VAN B. ROBERTS COUPLED CONSTANT AMPLIFICATION CIRCUITS FiledOct. 3}, 1932 Fig. 1

3 Sheets-Sheet l AAAAA no nuonOOOOOOO' 4 f g 2 i 6 "nun INVENTOR- WALTERVAN B. ROBERTS A'ITORN EY- Dc. 22, 1936. w. VAN B. ROBERTS CAPACITYCOUPLED CONSTANT AMPLIFICATION CIRCUITS Filedoct. 31, 1932 3Sheets-Sheet 2 INVENTOR- WALTER VAN B.ROBERTS BY ATTORN EY- Dec. 22,1936. w, VAN ROBERTS 2,064,990

CAPACITY COUPLED CONSTANT AMPLIFICATION CIRCUITS Filed Oct. 51, 1932 3Sheets-Sheet 3 m mam 3 fr 1 am 70 01/17 .L K J.

INVENTOR- WALTER VAN B. ROBERTS ATTORNEY Patented Dec. 22, 1936 TED sraA oFFIcE CAPACITY COUPLED CONSTANT AlVIPLIFICATION CIRCUITS tion ofDelaware Application October 31,

10 Claims.

My present invention relates to coupling systerns, and more particularlyto improved capacity coupling systems for tuned radio frequencyamplifiers which give substantially constant ampiification andselectivity over the entire broadcast range.

It is well known in the prior art that the output of a tube may becapacitatively coupled to the tuned input circuit of a second amplifierstage. In such a coupling system the coupling condenser forms a mutualimpedance between the plate circuit of the preceding tube and thesucceeding tuned circuit. The amplification in such a coupling system isindependent oi frequency if the resistance of the tuned circuit isindependent of frequency. But, if the resistance is constant so also isthe selectivity of the tuned circuit. In any actual tuned circuit thisis not the case, and the increase of resistance with frequency makessuch a capacity coupled system unsatisfactory.

Now, I have discovered novel methods of, and devised improved means for,combining such a capacity coupling network with means for introducinginto the succeeding tuned circuit an amount of negative resistancesufii'cient to reduce the total effective resistance of the tunedcircuit to a value which is sumciently low for good amplification andselectivity and suificiently constant to maintain both amplification andselectivity satisfactorily constant.

Hence, it may be stated that it is one of the prime objects of thisinvention to provide in combination with an amplifier tub-e, means for35 capacitively coupling the output circuit of the amplifier to aresonant circuit, and additional means for controlling the totaleffective resistance of said circuit, said additional means beingadapted to introduce into the resonant circuit a negative resistancewhich varies with frequency in such a way as to maintain the totalefiective resistance substantially constant, or to make it vary withfrequency in any other desired way.

Another important object of the present invention is to provide acondensive coupling network between an amplifier tube and a tuned radiofrequency circuit, means being provided for controlling the resistanceof the tuned circuit so as to maintain the said resistance independentof frequency.

1932, Serial No. 640,397

Another important object of the present invention is to provide acondenser coupling between two cascaded amplifier tubes, the secondamplifier having a resonant input circuit, wherein the amplifying powerof the one of said tubes and 5 the total effective resistance of saidresonant input circuit are uni-controlledly adjusted in opposite sensesby a single operating means.

Another object of the invention is to provide an amplifier systemwherein a preceding amplifier tube is capacitively coupled to theresonant input circuit of another amplifier tube, means being employedfor introducing into the resonant circuit an amount of negativeresistance sufficient to reduce the total efiective resistance of theresonant circuit to a value which is sufficiently low for goodamplification and selectivity and sufficiently constant to maintain bothamplification and selectivity satisfactorily constant, one of saidamplifying tubes and the means for producing negative resistance in saidresonant input circuit being both enclosed within the same glassenvelope.

Still another object of the present invention is to provide a novel typeof tube structure for use in connection with a 'capacitively coupledamplifier, the said tube structure comprising electron emitting means, ascreen grid, a control grid, and two anodes electrostatically shieldedfrom each other by a suitable structure of the screen grid, or insteadof employing twoanodes, alternatively employing two control gridselectrostatically screened from each other by a suitable construction ofthe screen grid and a screening means at the potential of the cathode.

And still another object of this invention is to provide a light batteryoperated portable radio receiver utilizing amplifier circuits comprisingcapacitively coupled tuned radio frequency amplifier stages, eachamplifier stage including a novel tube construction, and each of suchtube constructions providing an amplifier tube as well as a means forintroducing negative resistance into the resonant input or outputcircuit of each of such amplifier tube constructions.

Still other objects of the invention are to improve generally thesimplicity and efiiciency of condenser coupled amplifier circuits, andto particularly provide such an amplifier circuit which is not onlydurable and reliable in operation, but

readily manufactured and assembled in radio receivers.

The novel features which I believe to be characteristic of my inventionare set forth in particularity in the appended claims, the inventionitself, however, as to both its organization and method of operationwill best be understood by reference to the following description takenin connection with the drawings in which I have indicateddiagrammatically several circuit organizations whereby my invention maybe carried into effect.

In the drawings,

Fig. 1 shows diagrammatically a condenser coupled amplifier for analysispurposes,

Fig. 2 shows a similar circuit to be analyzed,

Fig. 3 diagrammatically shows a capacitively coupled amplifier circuitembodyng the present invention,

Fig. 4 shows a modification ofthe arrangement shown in Fig. 3,

Fig. 5 shows a novel tube construction adapted for use in connectionwith the present invention,

Fig. 6 shows diagrammatically another modified form of amplifier circuitembodying the present invention,

Fig. '7 is a modification of the arrangement shown in Fig. 6, andshowing a novel modified form of tube construction,

Fig. 8 shows a circuit employing a tube construction similar to thatshown in Fig. 'l but slightly modified,

Fig. 9 shows a modification of the arrangement shown in Fig. 8,

Fig. 10 shows diagrammatically a portable radio receiver embodying thepresent invention,

Fig. 11 shows still another modification of the present invention, and

Fig. 12 shows a modified form of the arrangement shown in Fig. 11.

Referring now to the accompanying drawings wherein like referencecharacters in the different figures designate corresponding circularelements, reference is first made to Fig. 1 which shows a simplecapacity coupling between an amplifier tube l and a second amplifiertube 2, the coupling being provided by means of the condenser K. Thecontrol grids of tubes I and 2 are negatively biased with respect to theoathodes by means of the usual grid biasing networks 3 disposed in thegrounded cathode leads of both tubes. Potential for the anode of tube issupplied from a source of anode potential B (not shown) through a radiofrequency choke coil 4, a direct current blocking condenser 5 beingconnected between the anode of tube ll and the coupling condenser K.

Between the control grid and cathode of the amplifier tube 2 isconnecteda resonant input circuit comprising the variable tuningcondenser C connected in series with the inductance coil L, theresistance of the resonant circuit being represented by a resistor 1"connected in series between the cathode and the inductance coil L. Theseries resonant path C, L, is connected in shunt across the condenser K,and the radio frequency voltage for the tube 2 is obtained by connectingthe control grid and cathode of the tube across the inductance L, theresistor 1" also being included in this input path.

Representing the radio frequency input to the tube 1 by the symbol E1,and designating the input to the tube 2 by E2, and assuming that theload impedance is so low that the tube may be considered as a generatorof constant cur rent, it can be readily shown that the voltageintroduced into the tuned circuit of tube 2 is:

l 'mg fi 5 The current at resonance is B G... m 10 and therefore isconstant. 7

As is well known, the voltage for the input of tube 2 may be taken offacross the tuning condenser C, and Fig. 2 shows such an arrangement, thevoltage E2 being taken across the variable tuning condenser C in orderthat the condenser rotor may be grounded. At the high frequency end ofthe tuning range of the condenser C the voltage across this condenserwill be almost exactly the same as across the coil L, but at the lowfrequency end it will be about fourfifths of the voltage across thecoil. Thus, Fig. 2 will give constant amplification only if r increasesapproximately twenty percent with frequency from the lowest to thehighest frequency of the tuning range of condenser C.

In any actual tuned circuit the resistance of the tuned circuit does notremain constant but increases with frequency. This increase ofresistance with frequency makes capacity coupled systems of the typesshown in Figs. 1 and 2 unsatisfactory. It is a well known fact that in acoil such as the coil L, the value of r is not a constant throughout thetuning range of condenser C. Therefore, the present inventioncontemplates combining such a capacity coupling network with means forintroducing into the tuned input circuit of the tube 2 an amount ofnegative resistance sufficient to reduce the total effective resistanceof the tuned circuit C, L to a value which is sufiiciently low for goodamplification and selectivity and suificiently constant to maintain bothamplification and selectivity satisfactorily constant. Without using thepresent invention there results in Figs. 1 and 2 a deviation inselectivity and amplification of about Negative resistance may beintroduced into a resonant circuit in several ways well known in theart.

By employing any of the various means disclosed in the presentapplication, there is obtained a coupling system for tuned radio frequency amplifiers which gives substantially constant amplification andselectivity over the entire broadcast band without employing any of thearrangements disclosed heretofore in the prior art. For example, withthe constants employed in the present invention the conductance of thecoupling system at resonance is so greatly in ex cess of that of theplate circuit of the preceding tube that it is not a case of acompromise between maximum amplification and maximum selectiv- 75 ity ascontemplated by Well known methods of the prior art. In my presentarrangement the amplification is so very far from the maximum possiblevalue, and the selectivity substantially that of the tuned circuitalone.

Fig. 3 shows one method of introducing negative resistance into thetunable input circuit of the amplifier tube 2. While a dynatron could beemployed for introducing the negative resistance into the tuned inputcircuit of tube 2, the arrangement shown in Fig. 3 shows an improvedmethod of using an auxiliary tube A whose sole function is to introduceregeneration into the tuned circuit C, L. Another way of stating thelatter function is that tube A serves to keep the total effectiveresistance of the resonant circuit K, C, L constant. The grid of tube Ais connected by a lead 6 to the grid of tube 2, and the anode of tube Ais connected to the grounded cathode lead through a series path whichincludes the coil T1, the coil T2 shunted by condenser K2, and the anodepotential source B.

The output of tube A is coupled to the coil L by a pair of couplings, inthis case by mutual inductance, between coils T1 and T2 and L. Thiscoupling is well known to the art as compound coupling, the couplingsdepending on the tubes and circuits used and being so chosen that thefeed-back, or negative resistance, introduced into the tuned circuit KCLvaries with frequency in such manner that the total effective resistanceremains approximately constant. As pointed out heretofore such anoperating condition necessarily results in the amplification andselectivity remaining substantially constant throughout the tuning rangeof the condenser C.

In Fig. 4 there is shown a modification of the arrangement shown in Fig.3 wherein there is employed a volume control device R, disposed in thegrounded cathode lead of tube 2, which acts to uni-controlledly vary theamplification of the amplifier tube 2 and the amount of regeneration ofthe regenerative tube A. Only so much of Fig. 3 is shown in Fig. 4 whichis essential to an understanding of the modification, it being pointedout that the cathode of the auxiliary tube A is connected directly tothe cathode of the amplifier 2. Hence, the bias on both tubes is alwaysthe same, and is controlled by the adjustable resistor R. It will benoticed that the cathode resistors R1 and R2 of Fig. 3 have beencoalesced into a single variable resistance.

Suppose, now, that tube A is operated at potential such that it isrelatively ineffective unless its bias is made less than the value whichcauses the amplifier tube to amplify reasonably well. Then, decreasing Rfrom large values toward a. certain minimum, there is obtained thefollowing sequence of performance characteristics. First, R is so greatthat the amplifier tube does not amplify, nor does the tube A produceappreciable regeneration. This condition would occur when listening tovery strong signals, when amplification is not needed, nor increasedselectivity. Second, R is reduced until a considerable amount ofamplification is obtained, but no appreciable amount or feed-back.Thirdly, R is reduced to a value where the feed-back becomes such as toreduce the total effective tuned circuit resistance to a value whichgives a desired selectivity, and also, in virtue of what has beendescribed in connection with Fig. 1, a desired maximum amplification.Thus, adjustment of R simultaneously alters both amplification andselectivity, making both approach an upper limit which may be designedto be independent of frequency.

Stated another way when the resistor R is adjusted to have a large valuethe amount of regeneration is negligible, and the amplification issmall, the fidelity being a maximum. Such an adjustment is suitable forreceiving local stations where fidelity rather than selectivity isdesired. When the resistance is made small, however, the regenerationbecomes sufficient to make the circuit highly selective and theamplification increases because the amplifier tube mutual conductanceincreases, and, additionally, because the tuned circuit becomes moreefiicient. This arrangement can be used to a special advantage inportable receivers having only one or two stages of amplificationbecause in such receivers an artificial increase in selectivity would beespecially valuable.

In practice, an auxiliary tube accompanying each amplifier tube wouldincrease the number of tubes in a receiver to an objectionable extentand, therefore, I contemplate the use of an armplifier tube which has anauxiliary plate, or an auxiliary control grid, for the purpose ofefiectlng the regeneration. Accordingly, there is shown in Fig. 5 avertical section of an electron discharge tube designed to combine bothamplification and regeneration in a single tube. The reference niunerall designates the glass envelope of the tube, the numeral 8 designatesthe cathode of the tube, the numeral 9 represents the control grid ofthe tube, one of the anodes being designated by the numeral l0 and theother anode by the numeral H. The smaller anode, or plate, I!) is theregenerating plate, and corresponds to the plate of tube A of Fig. 4.There is a screen H disposed around the main, or amplifying, plate H,but a screen is not absolutely required around the regeneration plate,although for the sake of completeness such a screen ID has been showndisposed around the plate 10.

The part of the grid 9 which controls the flow of electrons through theregeneration plate is made of wire 9 sufiiciently large, or spacingsumciently close, so as to preclude appreciable regenerativeamplification until the control grid bias has been reduced to a pointgiving a predetermined amplification. This refinement is not absolutelynecessary, as the feed-back couplings can be arranged so that themaximum desired regeneration is attained simultaneously with maximummutual conductance as the control grid bias is decreased.

In what has preceded, it has been assumed that the pair of tubes, orcomposite amplifier-regeneration tube, acts on its input circuitregeneratively, and produces amplified voltage in its output circuit.This system is satisfactory for each tube has a tuned input circuitwhich it may regenerate, the detector regenerating its input, while theantenna and radio frequency tubes act as the sources of signals whichare coupled to the thus regenerated circuits. However, in somecircumstances it may be preferable to have each pair of tubes, orcomposite tube, act on the same circuit. Hence, there is shown in Fig. 6an arrangement for securing such an operation, and it will be noted thatFig. 6 shows a modification of Fig. 4 wherein the amount of regenerationis uni-com trolled along with the mutual conductance of the tube whichfeeds into the circuit regenerated.

In Fig. 6, then, the tuned input circuit of tube 2 is capacitivelycoupled by the condenser K to the anode circuit of the precedingamplifier tube I. It will be observed that the arrangement shown in Fig.6 is similar to the arrangement of Fig. 3, with the exception that theadjustable resistor R, is connected in the grounded cathode lead of tubethe high potential side of the resistor B being connected by a lead 6'to the cathode of the tube A. The negative terminal of the potentialsource 13' is grounded, and the manner of coupling the coil L, the coilT1 and the coil T2 is shown. The tube i is connected to the source ofhigh frequency energy having a potential E1 to be amplified, and thetube i may be the first radio frequency amplifier of a receiver, forexample. As pointed out heretofore, in the modification shown in Fig. 6the amount of regeneration of the tuned circuit K, C, L issimultaneously controlled with the mutual conductance of the tube iwhich feeds into the tuned circuit which is being regenerated.

There is shown in Fig. 7 an arrangement embodying a composite tubecombining the functions of the tubes i and A of Fig. 6. In thiscomposite tube, as in the case of the tube shown in Fig. 5, there isdisposed within the common envelope l, the cathode 8, the smallauxiliary regeneration anode it, and the larger amplifying anode ll,both of the anodes being surrounded respectively by the screens iii andii. The amplifier control grid ii is shown separate however from theregenerator control grid 53', and screen 9" is disposed between thecontrol grids 9 and 9' and functions to prevent capacity couplingbetween control grid 9 and the tuned output circuit of the tube 1. Thegrounded lead of the cathode 8 includes the adjustable resistor R,shunted by the high frequency by-pass condenser l2, it being understoodthat the resistor R functions in the same manner as described inconnection with Fig. 6.

The tube shown in Fig. '7 may be simplified, however, by employing asingle anode I3, as shown in Fig. 8. This may be done while main taininguni-control of the amount of the regeneration and the variation ofregeneration with frequency, if the properly chosen compound coupling isemployed, not to feed back the voltage to the tuned circuit, but to pickup the voltage from the tuned circuit to apply to the regeneration grid9 through a lead l connected between the grid 9 and one side of the coilT1. It will beobserved that this differs from the connection in Fig. '7wherein the grid 9' is connected by a lead M to the high potential sideof the coil L, whilethe regeneration anode ill is connected by a lead I4to one side of the coupling coil T1. It will benoted that a screen i3 isdisposed around the: single anode l3, and that the screen 9" is disposed between the grids 9 and 9.

In the arrangements described above, the network including coil T1, coilT2, and the shunt condenser K2 has been used merely as illustrative: ofany desired method for obtaining an effective mutual impedance havingsuitable variation with. frequency. It is possible, however, to obtaingood results with the compound coupling by arranging the geometry of theregeneration grid 9 of' the tube shown in Fig. 8 so that no appreciableregeneration is introduced until the value of the resistor R is reducedto a point where the mutual. conductance of the amplifying part of thetube isnearly a maximum. In this way the single volume control R, if itsresistance is reduced, first increases amplication in normal fashion,then increases selectivity with a resultant further increase inamplification.

Such an arrangement is shown in Fig. 9. In this figure the renegerationgrid 9 is indicated as of a closer mesh than the amplifying grid 9 sothat regeneration is small if R is large. Also, a metallic disk 8 ofconducting metallic material around the cathode opposite the shielding 9between the grids is indicated, to prevent unnecessary screen current.The regeneration grid 9 is tapped onto the tuned circuit, by a lead IE3,at a point where, with R at the minimum safe value, regeneration is asstrong as desired at all frequencies, and with tubes as poor asconsidered fit for sale. With extra good tubes, and/or at somefrequencies, oscillation will result from advancing the volume controltoo far. However, it is believed that this is a desirable possibility,rather than otherwise, in cheap sets having few tubes. If the user doesnot like oscillations he does not need, of course, to produce them. Aloop antenna, followed by two stages of amplification such as shown inFig. 9, and a pentode high power detector directly feeding into aloudspeaker should make an excellent portable set.

Such a portable light battery operated receiving arrangement isdiagrammatically shown in Fig. 10 wherein the tubes 2!], 2!, 22 are tobe understood as being of the type shown in Fig. 5, the regenerationanode 23 of tube 25 being coupled, as at L, to the tunable input circuitof tube 28. The input circuit is preferably a loop antenna comprising acoil 25 shunted by a fixed condenser K1 in series with a variable tuningcondenser 25. The fixed condenser K1 is shunted by the secondary of thecoupling transformer L, and the regeneration anode lead 23' includes theprimary coil L1 of the coupling transformer L in series with a coil 26,inductively coupled to the coil 24, and having one terminal thereofconnected to the positive terminal of a source of anode potential B (notshown).

The amplifier anode of tube is connected to the positive terminal of asource of anode potential through the primary coil 21, the latter beingcoupled to a secondary coil 28 shunted by a condenser coupling 29 whichfunctions in a manner similar to the coupling condensers K of thevarious figures. The condenser 29 is shunted by the variable tuningcondenser 30, a coil 3! being connected between the high potential sidesof the condensers 29 and 30. The cathodes of tubes 20 and 2! areconnected to ground through a radio frequency by-pass condenser 32, andthe rotors of the tuning condensers 25, 36, as well as the rotor of thedetector tuning condenser 40, are grounded so that they may besimultaneously adjusted by a common mechanical uni-control mechanism 4|.

Theregeneration anode 2| of tube 2| is tapped onto a point on coil 2'!through a lead 33 which includes the coil 34 coupled to the coil 3!. Thecommon cathode lead of tubes 20 and 2!, also, is connected to groundthrough a path including the resistor R2 in series with the adjustableresistor R employed for varying the mutual conductance of tubes 20 and2!, as well as the variation of regeneration of these two tubes. Thecoupling network between the amplifier anode circuit of tube 2! and theinput electrodes of detector tube 22 is similar to the coupling networkarrangement shown between tubes 26 and 2i,

detection, although it is to be clearly understood that the detector maybe operated with a leaky grid condenser instead.

The reference numeral 5| is to be understood as representing any desiredtype of reproducer adapted to be disposed in the amplifier anode circuitof detector tube 22. Of course, as is well known in the art, a commonsource of potential B may be provided for the various circuits of tubes26, 2E and 22. Furthermore, there may be employed in the tube 22 aso-called suppressor grid; that is a grid connected to the cathodewithin the tube may be disposed between the shield for the two anodesand the two anodes themselves, such a pentode tube suppressor grid beingwell known in the art.

The receiver shown in Fig. 10 operates in the following manner.Regeneration anode 23 is coupled to the resonant input circuit by asimple untuned inductance between loop 24 and coil 26, and by a capacitycoupling K1 connected into the anode circuit by way of transformer LL.This transformer allows segregation of steady potentials while allowingthe coupling as a whole to be equivalent to the simple mutual capacitycoupling shown in Fig. 1 provided the natural frequency of circuit K1Lis low compared to signal frequency. This compound coupling betweenregeneration anode 23 and the input circuit is arranged to keep theeffective resistance of the input circuit low and independent offrequency when R is kept =0.

The relay anode of tube 20 is capacitively coupled to the tuned inputcircuit of tube 2| by the transformer 2T-28 in combination with couplingcondenser 29 as explained above. The regeneration anode 2l' of tube 2|is however compound coupled to the resonant input circuit of tube 2!,the magnetic coupling being the mutual inductance between coils 34 and31 while the capacity coupling means is a portion of that coil 21 whichcouples the main anode of tube 20 to the resonant input circuit of tube2|. The main anode of tube H is coupled to the resonant input circuit ofdetector 22, and this resonant circuit is regenerated, in exactlysimilar fashion. Resis tance R. controls both the amplification andregeneration of the first two tubes as described in connection withother figures.

In the arrangements described up to this point, regeneration has beenassumed to be obtained by a separate tube, or a separate electrode in ascreen grid tube. However, if a screen grid tube is used, the feed-backcoil may be put in its plate, or screen circuit. Thus, in Fig. 11 thescreen grid tube 68 has its anode circuit capacitively coupled to thetunable input circuit of tube 6!, the input electrodes of tube 6! beingconnected across the tuning condenser 62. This coupling network has beenshown in Fig. 2, and need not be described in any further detail. Itshould be noted that the anode of tube is regeneratively coupled to thetunable input circuit coil 63 by means of a coil 64.

The reason Why the arrangement of Fig. 11 is a possibility with screengrid tubes is that radio frequency potentials on the plate of tube 60are prevented from appreciably aifecting the grid thereof through theplate-grid capacity, and the load impedance is so small relative to theplate impedance that no appreciable reduction in the current throughtickler coil 64 is caused by tuning the load circuit. It is preferableto locate the tickler coil in Fig. 11 at the low potential end of thegrid coil to minimize capacity coupling.

Fig. 12 shows a modification wherein the coil 64 is disposed in thescreen grid circuit of the tube 60. The arrangement of Fig. 12 ispossible because as there is very little load in the circuit of ticklercoil 64, very little radio frequency potential is built up on thescreen. Hence, maximum feed-back occurs at exact resonance tuning of theinput, and the screen potential being nearly constant allows the screento carry out its normal function of screening around the plate.

While I have indicated and described several systems for carrying myinvention into effect, it will be apparent to one skilled in the artthat my invention is by no means limited to the particular organizationsshown and described, but that many modifications may be made withoutdeparting from the scope of my invention as set forth in the appendedclaims.

What I claim is:

1. In combination, an amplifier tube provided with an output circuit, asucceeding resonant circuit adapted to be tuned throughout apredetermined frequency range, means for coupling the output circuit ofsaid tube to said resonant circuit comprising a fixed condenser commonto said two circuits, and means independent of the coupling means forcontrolling the total effective resistance of said resonant circuit.

2. In combination, an amplifier tube provided with an output circuit, asucceeding resonant circuit adapted to be tuned throughout apredetermined frequency range, means for coupling the output circuit ofsaid tube to said circuit comprising a fixed condenser common to saidtwo circuits, and means independent of the coupling means forcontrolling the total effective resistance of said resonant circuit to avalue which is sufiiciently low for good amplification and sufficientlyconstant to maintain both amplification and selectivity satisfactorilyconstant.

3. In combination, an amplifier tube provided with an output circuit, asucceeding resonant circuit adapted to be tuned throughout apredetermined frequency range, means for coupling the output circuit ofsaid tube to said circuit comprising a fixed condenser common to saidtwo circuits, means independent of the coupling means for controllingthe total effective resistance of said resonant circuit, and a singleoperating means for uni-controlledly adjusting the amplifying power ofsaid tube and the total effective resistance of said resonant circuit.

4. In combination, an amplifier tube provided with an output circuit, asucceeding resonant circuit adapted to be tuned throughout apredetermined frequency range, means for coupling the output circuit ofsaid tube to said circuit comprising a fixed condenser common to saidtwo circuits, and means independent of the coupling means including anelectrode connected to a feedback circuit coupled to the resonantcircuit for controlling the total effective resistance of said resonantcircuit, said electrode being disposed within said amplifier tube.

5. A radio receiver comprising at least two cascade radio frequencyamplifier stages and a power detector stage, each of said stagescomprising input and output circuits and a tube which has in addition tothe usual cathode, control grid and anode, an auxiliary anoderegeneratively coupled to the input circuit of its respective stage, anda common fixed condenser for coupling the input circuits of the stagesto the respective output circuits of the preceding stages, and a commonmeans for controlling the amplifying power of the two amplifier stagesas well as the regeneration of at least one of the stages.

6. In combination a circuit including a tube having an input circuit anda plate circuit, a condenser tuned resonant circuit, means including amutual impedance for coupling the plate circuit to said tuned resonantcircuit, said mutual impedance being substantially proportional to Wavelength over the tuning range, and means for automatically addingnegative resistance to said tuned circuit in amount which increases withincreasing frequency, While maintaining said resonant circuitsubstantially uncoupled from said input circuit except by the repeateraction of the tube.

7. An interstage coupling arrangement for obtaining uniformamplification and selectivity of signals transferred between a pair ofcascaded Vacuum tube stages comprising a tunable circuit including avariable condenser, an inductance and a fixed condenser connected toform a closed circuit, the output electrode of the first stage beingcoupled to the fixed condenser, the input electrode of the second stagebeing energized from said tunable circuit, and means coupled to saidinductance for controlling the effective resistance of said tunablecircuit.

8. An interstage coupling arrangement for obtaining uniformamplification and selectivity of signals transferred between a pair ofcascaded vacuum tube stages comprising a tunable circuit including avariable condenser, an inductance and a fixed condenser connected toform a closed circuit, the output electrode of the first stage beingcoupled to the fixed condenser, the input electrode of the second stagebeing energized from said tunable circuit, and means for controlling theeffective resistance of said tunable circuit comprising an auxiliarytube having its input electrode connected to the resonant circuit andits output circuit regeneratively coupled to the resonant circuit.

9. An interstage coupling arrangement for obtaining-uniformamplification and selectivity of signals transferred between a pair ofcascaded vacuum tube stages comprising a tunable circuit including avariable condenser, an inductance and a fixed condenser connected toform a closed circuit, the output electrode of the first stage beingcoupled to the fixed condenser, the input electrode of the second stagebeing energized from said tunable circuit, and means for controlling theeffective resistance of said tunable circuit comprising an auxiliarytube havingits input electrode connected to the resonant circuit and itsoutput circuit regeneratively coupled to the resonant circuit, andsimultaneously variable cathode resistors for said auxiliary tube andthe tube of the first stage.

10. An interstage coupling arrangement for ob taining uniformamplification and selectivity of signals transferred between a firstvacuum tube provided with at least an output electrode and an auxiliaryelectrode and a second vacuum tube provided with at least an inputelectrode, comprising a tunable circuit including a variable condenser,an inductance and a fixed condenser connected to form a closed circuit,the output electrode of the first tube being coupled tothe fixedcondenser, the input electrode of the second tube being energized fromsaid tunable circuit, and means for controlling the effective resistanceof said tunable circuit comprising the auxiliary electrode contained inthe tube of the first stage and a feedback circuit connected theretoinductively coupled to the tunable circuit.

WALTER VAN B. ROBERTS.

